Wire drawing dies made with natural diamonds have two disadvantages. One is that natural diamonds are not as strong as synthetic diamonds and there is risk of the diamond breaking under the force required for drawing wire. Much more important disadvantage is that natural diamonds are crystalline and do not wear evenly. This requires drilling to a larger size die after an unpredictable length of time during which the die wears unevenly so that it is no longer suitable for producing wire of round cross section and of the original diameter for which the die was made.
Polycrystalline synthetic diamonds wear like amorphous materials and their wear is even and predictable. The lack of cleavage lines practically negates the high degree of breakage encountered with natural diamonds.
The synthetic diamond die of this application has a throat of substantially uniform diameter throughout its length and with wearing surface that maintains contact with the circumference of the wire throughout substantially the entire length of the throat. Beyond each end of the throat there are tapered walls which differ in their angle of taper depending upon the material of the wire with which the die is intended to be used. These tapered passages, at opposite ends of the throat are referred to herein as approach passages and the one at the upstream end of the throat reduces the cross section of the wire as the wire moves toward the throat. The tapered passage at the downstream end of the throat constitutes the approach passage when the wire is being drawn through the die in the opposite direction. The throat is substantially cylindrical and equal in diameter to the downstream diameter of the approach passages in which the reduction in wire diameter is effected. By using the die in such a way that each end of the die is the upstream end substantially half of the time that the die is in use, the wear on the approach passages is much less and the service life of the die substantially twice as long as it would otherwise be. Experience has shown that such is the case.
The taper of the passages as they approach the throat are referred to as the "drawing angle." At the ends of the tapered passages, there is a short length of each passage which has a larger angle of taper and these ends with the larger angle of taper are referred to as the "bell" of the passage.